In U.S. Pat. No. 5,549,380, a bone cement mixer having a flexible liquid container for monomer, is described. Flexible monomer containers consist of a flexible wall material which provides for their flexibility, and a metal layer, preferably an aluminum layer, prevents the monomer from penetrating the flexible wall material. Thus, the walls of the monomer container consist of several layers, which means that the container is expensive to manufacture. Also, it is difficult to fill flexible monomer containers with monomer and to seal and open the container. Furthermore, flexible monomer containers are difficult to store, transport and handle and they are also sensible to blows and bumps.